Bone is, together with kidney, the major target organ of PTH. It is known that PTH increases bone turnover, but it is obscure why in vivo it has an anabolic effect when it is administered intermittently, while its catabolic action prevails when it is given in a continuous fashion. PTH augments osteoclastic activity, but it is controversial whether its effects on osteoclasts are only mediated through osteoblasts, or whether they are also direct. Osteoblasts are the major target of PTH action. Which cells, however, in the osteoblast lineage- a spectrum from osteoprogenitors to pre-osteoblasts and mature osteoblasts - mediate the various actions of PTH in vivo is an open question. It is also unknown how PTHrP influenced osteoblast function independently of its role on chondrocyte differentiation. Finally, the cloned PTH/PTHrP receptor is able to activate at least two different signaling pathways, adenylate cyclase and phospholipase C. However, their relative importance in mediating the PTH effect on osteoblasts needs to be clarified. Recently the identification of two different mutations in the PTH/PTHrP receptor in patients with Jansen's metaphyseal chondrodysplasia (JMC), that lead to ligand-independent constitutive cAMP accumulation, gave us a powerful tool to explore experimentally at least some of the above mentioned questions. We propose to develop transgenic mice in which mutant PTH/PTHrP receptors will be expressed under the control of two different promoters. alpha1(I) collagen and osteocalcin promoters, to allow their targeted expression in less mature and/or more mature osteoblasts respectively (Specific Aim I). The analysis of these animals will help to determine and distinguish the roles of these cells in mediating the response to PTH and PTHrP (Specific Aim II). We also propose to mate these transgenic mice with the ones that selectively lack the PTH gene, to identify PTH effects on osteoblast function and maturation that are dependent on the PTH/PTHrP receptor activation of the cAMP pathway (Specific Aim III). Lastly, we will mate the above described transgenic mice with the animals that are resistant to collagenase-3 cleavage to confirm the importance of collagenase-3 induction for the bone remodeling process induced by PTH, both during development and in adult life (Specific Aim IV).